Recently, a variety of apparatuses of this type have been developed and the present application also has already proposed one such apparatus. These apparatuses are designed so that signals from sensors which detect the operating conditions of an engine cause the on-off operation of a solenoid valve disposed in a fuel pressure control circuit, thereby changing the pressure difference across a fuel measuring gate to compensate the air-fuel ratio according to the operating conditions of the engine. If the output time ratio between rich and lean signals from said sensors (which corresponds to the on-off time ratio of the solenoid valve) deviates from a predetermined value, the pressure in the bellows of a servomechanism which detects the air flow rate is changed by a heater so as to correct the basic air-fuel ratio, which is set by said servomechanism, to maintain said time ratio at a predetermined value, thereby maintaining said air-fuel ratio at a desired constant value while reducing the time required to effect said compensation of the air-fuel ratio to suit it to the operating conditions of the engine and improving the response characteristic of the engine.
However, there are cases where the bellows of the servomechanism has to be reduced in size depending upon the design conditions thereof, and the operation of installing said heater in such bellows is not easy. Further, it requires a high degree of skill to insulate the heater and bellows from each other and at the same time seal the bellows to prevent leakage of the gas contained therein. Thus, it has been desired to improve said arrangement.
Accordingly, in view of the above described drawbacks of the apparatus previously proposed by the present applicant, it is an object of the present invention to provide an improved apparatus.